Background: The Truview EVO2 blade facilitates the view of vocal cords by indirect laryngoscopy and does not require the proper alignment of the oral, pharyngeal and tracheal axes as with the Macintosh blade. Methods: In a crossover fashion, we prospectively compared the view obtained at laryngoscopy with Truview EVO2 and the Macintosh blade in 110 adult patients of either sex between the age of 18 and 60 years, who were scheduled to undergo general anesthesia with endotracheal intubation. The patients were intubated with the second laryngoscope. The preoperative airway variables, laryngoscopic view, difficulty of intubation scale (IDS) score, duration of intubation, and degree of difficulty percentage of glottic opening (POGO score) of use with each laryngoscope were compared. Results: The IDS score was low and comparable between the two laryngoscopes. The laryngeal view was easy; Modified Cormack Lehane (MCL) grade 2a or less in 98.14% of the cases with the Truview laryngoscope compared to 78.7% of the cases with the Macintosh laryngoscope. Nineteen patients of MCL grade 3, one patient of grade 2b, and seven patients of grade 2a view with the Macintosh laryngoscope had MCL grade 1 view with the Truview laryngoscope. The duration of intubation was comparable between Truview and Macintosh laryngoscopes (12.1±3.8 s vs. 10.9±2.1 s). Conclusion: Truview laryngoscope performed comparably to Macintosh laryngoscope in patients with normal airway; however, the Truview laryngoscope may be a better option in difficult airway situations when the Macintosh blade fails to show the glottic opening.

The curved laryngoscope blade described by Macintosh in 1943 is the most popular device used to facilitate orotracheal intubation and constitutes a gold standard. [1] In the recent past, many different designs of laryngoscopes have been developed in an effort to reduce the incidence of unanticipated difficult airway as it is associated with serious morbidity and mortality. [2],[3],[4],[5] Difficult intubation can be in the form of inability to visualize the larynx using a conventional blade or it can be in the form of difficulty in passing the tube into the trachea despite the visualization of the vocal cords. In these circumstances, the airway is usually secured either with a laryngeal mask airway or with fibereoptic devices, which allow an indirect view of the vocal cords. [6]

The Truview EVO2 laryngoscope (Truphatek International Ltd, Netanya, Israel) is designed to circumvent the problem of poor view at laryngoscopy, which is the most common problem during difficult intubations. The Truview blade is a modified laryngoscope blade incorporating an unmagnified optic side port. The optical apparatus provides a 42° angled deflection view through a 15-mm eyepiece [Figure 1]. The angle of view facilitates vision in patients with limited neck extension. The Truview eyepiece can be connected to an endoscopic camera head with a monitor. In addition, the Truview blade has a port that connects to the auxiliary oxygen flow meter of the anesthesia machine (flow rate of 4-6 L/min), which prevents misting, clears secretions from the lens, and provides continuous oxygen insufflation during intubation. Many studies have shown that the Truview blade may be a better option than the Macintosh blade in the management of normal and difficult airway utilizing Modified Cormack Lehane (MCL) grading. [7],[8],[9],[10],[11],[12] The aim of this study was to assess the usefulness of the Truview EVO2 laryngoscope for use by experienced anesthetists in routine adult airway management utilizing the intubation difficulty scale (IDS) score. The IDS score developed by Adnet et al. is a quantitative scale of intubation difficulty that can objectively compare the complexity of tracheal intubations. According to this, "ideal" intubation i.e., one performed without effort on the first attempt, practiced by one operator, using one technique, with full visualization of the laryngeal aperture and vocal cords abducted, is given an IDS value of zero. Each variation from this defined "ideal" intubation increases the degree of difficulty and overall score value. [13]

Following approval of the Institute Ethical Committee (Post Graduate Institute of Medical Education and Research, Chandigarh, India), 110 adult American Society of Anesthesiologists (ASA) physical status I patients of either sex, aged 18-60 years who were scheduled to undergo an elective surgical procedure that required general anesthesia with oral endotracheal intubation were included in the study. Signed informed consent was obtained from each patient who agreed to participate in the study. Patients were not included if they had Modified Mallampatti (MMP) class III and IV (MMP Class 1-uvula, fauces, tonsillar pillars, and soft palate visible; Class II-uvula fauces and soft palate visible; Class III-soft palate and base of uvula visible; and Class IV-cannot see even the soft palate). [14] Patients were also excluded if they had a thyromental distance less than 6.5 cm, inter-incisor distance less than 4 cm, history of difficult airway, cervical spine injury, risk of regurgitation, i.e., full stomach, emergency surgery, pregnant patient, and obesity Body Mass Index (BMI)>30 Kg/m 2 . All the patients received oral diazepam (0.1 mg/kg), ranitidine 150 mg, and metoclopramide 10 mg the night before and on the morning of surgery. In the operation theatre, the patients were continuously monitored using an electrocardiogram, pulse oximeter, end tidal carbon dioxide, and an automatic non-invasive blood pressure device. Intravenous line was secured and normal saline was started. After pre-oxygenation with 100% oxygen, anesthesia was induced with intravenous morphine 0.1 mg/kg along with propofol 2-3 mg/kg. Neuromuscular blockade was achieved by using vecuronium bromide 0.1 mg/kg. Laryngoscopy was performed 4 min after vecuronium administration. Laryngoscopy was performed twice in each patient using in turn both the Macintosh laryngoscope size 3 (Vision Medical Devices Pvt. Ltd, India) and Truview laryngoscope (adult) in a random order generated from the computer. Each laryngoscopy was performed in the sniffing position. View of the larynx with both blades was recorded based on MCL grading. Grading criteria (grade 1-full view of vocal cords; grade 2a-partial view of vocal cord; grade2b-arytenoids and epiglottis visible; grade 3-only epiglottis is visible; and grade 4-both epiglottis and glottis not visible), and the percentage of glottic opening (POGO) score (0% when glottis is not visible and 100% when the entire glottis is visible). [15],[16] No laryngeal pressure was applied to improve this score. When required, anterior laryngeal pressure was applied to facilitate orotracheal intubation. Trachea was intubated after second laryngoscopy using an appropriate sized cuffed endotracheal tube preloaded with stylet. The degree of difficulty in intubation was based on the IDS score [Appendix 1]. [Additional file 1]

Once the intubation is complete, the lungs were mechanically ventilated for duration of the procedure and anesthesia was maintained using 66% nitrous oxide with oxygen and halothane (0.5-1%). The primary endpoint was the IDS score. Time for successful intubation was also noted. The duration of intubation was defined as the time taken from insertion of the second blade between the teeth until the Endotracheal tube (ETT) was placed through the vocal cords, as confirmed visually by the anesthetist. If the endotracheal tube was not visualized passing through the vocal cords, the intubation attempt was not considered complete until the ETT was connected to the a breathing circuit and evidence obtained of the presence of CO 2 in the exhaled breath (capnograph). The secondary endpoint was the rate of successful placement of the tracheal tube in the trachea. A failed intubation attempt was considered when the trachea was not intubated or the time required for intubation was more than 120 s. Once tracheal intubation was successfully accomplished, the anesthetist scored the ease of use of each device using a visual analogue scale (from 0=extremely easy to 10=extremely difficult).

Data were analyzed using the Statistical Package for Social Sciences version 11. We based our sample size estimation on the IDS score. An IDS score of 0 represents ideal intubating conditions and increasing score represents progressively more difficult intubating conditions. Based on the study by Maharaj et al., we have projected IDS scores of ≥1 in 50% of these low-risk patients with the Macintosh laryngoscope. We considered that a clinically important increase or decrease in the number of patients with an IDS score ≥1 in these low-risk patients would be a 50% absolute change (increase or decrease), i.e., an IDS score of ≥1 in 25% of these patients. Based on these figures, using an α=0.05 and a β=0.2 for an experimental design incorporating two equal-sized groups, we estimated that 53 patients would be required per group. Therefore, 55 patients were enrolled per group and a total of 110 patients were included in the study.

The demographic data was analyzed using the 't-test'. The data for the laryngoscopy and the intubation difficulty were analyzed using multiple tests. The Chi-Square test was used for assessing the MCL grading for ease of laryngoscopic view, the t-test was used for the Visual Analogue Scale, the POGO score, and the duration of intubation with each laryngoscope. The Mann-Whitney test was used for the IDS score along with the number of intubation attempts with each laryngoscope. A P0≤0.05 was considered statistically significant.

Results

A total of 110 patients were entered into the study. Fifty-five patients were randomly allocated to undergo tracheal intubation using the Macintosh laryngoscope and another 55 patients were to undergo intubation using the Truview laryngoscope. There were two failed intubations due to technical reasons rather than due to failure of the laryngoscope or the anesthetist to secure the airway. In the first case, a defective stylet was used, which could not conform to the shape of the tracheal tube, and in the other there was improper anti-fogging due to disconnection of the oxygen tubing from the anesthesia machine. Therefore, these two cases were excluded from the statistical analysis and data of only 108 patients were analyzed.

ASA physical status, mean age, and sex ratio distribution of the patients were comparable in two groups. There was a significant difference in the BMI (P=0.046) of patients in both the groups; the group in which the Truview blade was used had a higher BMI than the Macintosh group [Table 1].

Preoperative airway assessment variables such as mouth opening, mentohyoid distance, and MMP grade were comparable between the two groups. The mentothyroid distance was significantly greater ( P=0.038) in the Macintosh group (6.9±0.3 cm) compared to the Truview group (6.8±0.1) [Table 1].

IDS score of 0 in the Truview group (98.1%) was comparable to the Macintosh group (92.5%), which suggested easy intubation with both the laryngoscopes [Table 2]. The laryngeal view was easy (MCL grade 2a or less) in 98.14% of the cases with the Truview laryngoscope compared to 78.7% of the cases with the Macintosh laryngoscope. In patients with MCL grade 3 using the Macintosh laryngoscope, there was a definitive improvement in view when the Truview laryngoscope was used. The POGO score was comparable in both the groups. The average time of intubation was slightly more with the Truview laryngoscope (12.1±3.8s) compared to the Macintosh laryngoscope (10.9±2.1s); however, the difference was not statistically significant. The Visual analogue scale VAS score used to assess the ease of use of the laryngoscope showed no statistically significant difference between both the laryngoscopes.

This prospective randomized controlled trial was conducted to assess the utility of the Truview laryngoscope in normal adult airway. The difficulty of intubation was assessed with the IDS score. The IDS score was low and comparable between the two groups; hence, the intubation was easy with both the laryngoscopes. No manoeuvres were required to improve the success of intubation in either of the groups. In our study, first attempt intubation success was 100% with both the laryngoscopes; this may be because we did not include patients with anticipated difficult airway, used a stylet in both the groups, and an experienced anesthetist performed the laryngoscopy.

The parameters used to assess the laryngoscopic view were MCL grading and POGO score. Our study demonstrated that the laryngoscopy view that was assessed by MCL grading and the POGO score were better with the Truview laryngoscope compared with the Macintosh laryngoscope, although this difference was not statistically significant. Many authors have found similar results. Barak et al. showed that the MCL grading was significantly lower in the Truview group when compared to the Macintosh group. [9] Similarly, Leung et al. also demonstrated an easy laryngeal view (MCL grade 2a or less) in 60% of the cases with the Truview laryngoscope compared with the Macintosh and the McCoy laryngoscopes. [7] In the study conducted by Li et al., 105 out of 120 (87.5%) patients showed improvement in the MCL grading when Truview blade was used compared to the Macintosh laryngoscope. [8]

In addition to the MCL grading, the POGO score was also included as a measure of the laryngeal view in our study. POGO score is more sensitive than the MCL grading, and has been shown in previous studies to have a good intra-and inter-observer reliability. [17] The POGO scores of both groups were comparable. In a Manikin study, Leung et al. also utilized the POGO score to assess the laryngeal view at intubation and showed significantly higher POGO score in patients with the Truview laryngoscope compared to both the Macintosh and the McCoy laryngoscopes. [7] Timanaykar et al. also reported a higher POGO score (97.26±8) with Truview than with the Macintosh blade (83.70±21.5). [12]

The time taken for intubation was more with the Truview laryngoscope (12.1±3.8 s) than with the Macintosh laryngoscope (10.9±2.1s); however, this difference was not statistically significant. Similar results were obtained by Li et al., Barak et al., and Timanaykar et al. [8],[9],[12] The prolonged time of intubation in the various studies conducted with Truview has been explained by the indirect method of viewing the larynx through this optical laryngoscope. It is difficult to direct the tracheal tube through the vocal cords whilst viewing the image through the view tube. The field of vision is narrower and smaller, requiring more time to identify the pharyngeal and laryngeal structures. There is also an angulated view of the larynx, which necessitates the use of stylet to direct the tracheal tube to the glottic opening. Moreover, the experience of the anesthetist with the Truview blade is comparatively less compared to that with the Macintosh blade. The ease of use of laryngoscopes showed that both the blades were easy to use. There was no incidence of any dental or other trauma in either group.

The main limitation of our study was that, firstly, the anesthetist performing the intubation was not blinded to the study group due to the unfeasibility of blinding and the possibility of bias existed. Secondly, the experience of the anesthetist with the Macintosh laryngoscope was far more and better than that with the Truview laryngoscope. Thirdly, there were a few differences between the two groups with regard to the preoperative demographic data (BMI was significantly higher in the Truview group) and the airway assessment parameters (mento thyroid distance was significantly less with the Truview group).

In conclusion, the Truview EVO2 laryngoscope performed comparably to the Macintosh laryngoscope in the normal adult airway; however, the Truview laryngoscope may be a better option in difficult airway situations when the Macintosh blade fails to show the glottic opening. Additional studies are necessitated to prove the usefulness of Truview EVO2 in difficult airway situations, as this is the cheapest available video laryngoscope in the market.